Pia my heart goes out to you for feeling so sad. I agree that doing something to honour your father would be lovely and help you express the grief. You could write him a letter, have a huge cry and tuck the letter away somewhere. Even tell him your troubles.
I'm glad your mother is moving out if she's started playing games. You have a much better chance of a reasonable relationship if you aren't on top of each other.
If the ex isn't answering your text there must be a way of decoupling your card from the toll fees. Talk to the toll people and see what's possible. Am I correct in thinking the vehicle is still registered to you but he's driving it? What are the implications of that for you? This is something you can control (I hope) by taking decisive action, and that will help with the depression.
My theory about depression is that it can stem from a feeling of powerlessness or lack of closure. It might be things you've left undone for yourself, or power someone else has over you. Have a think about it; is there anything you can control that you need to finish?
If you're sinking too deep into depression please talk to your doctor. It will help to have someone to share your troubles with.